1. Field of the Invention
This invention relates generally to a signal reproducing circuit for reproducing a signal read out from a moving record carrier, and more particularly to a gain control circuit for controlling the gain of a circuit included therein for amplifying a signal from a record carrier in response to a position on the carrier where the signal is read out.
2. Description of the Prior Art
In the art then exist various types of signal recording carriers and apparatus for reproducing signals from these signal record carriers. One of these record carriers includes, a rotatable disc, on which a signal, such as a frequency-modulated (FM) video and audio signal, is recorded in a spiral track which travels from the periphery to the inside portion of the disc. The spiral track is formed with a plurality of small pits or bumps in alignment along the rotational direction of the disc and on the surface of the disc. The spaces between respective adjacent pits or bumps on the spiral track are varied in response to the frequency-modulated signal to be recorded. In the case of a video signal, one revolutionary path of the spiral track is typically given a recorded signal corresponding to one frame of the video signal. The length of each path or circle of the spiral track is different from the length of the next adjacent path or circle, that is the length of one circle elongates gradually to the peripheral portion from the central portion of the disc. This results in the dimensions of the pits or bumps and the space between respective adjacent pits or bumps on each circle of the spiral track being gradually increased toward to the peripheral portion from the central portion of the disc.
With the above-described record disc, one method for reproducing signals therefrom is by use of a laser light beam. In the reproduction, the recorded disc is rotated and the laser light beam of a constant diameter is impinged on the surface of the rotating disc so as to trace the spiral track on which the signal is recorded in the form of the alignment of pits or bumps. The laser light beam tracing the spiral track is modulated to have variations in accordance with the alignment of pits or bumps and reflected there to be directed to a photo-sensing device. The photosensing device generates a reproduced signal in response to the variations in the reflected laser light beam.
As mentioned above, each dimension of the pit or bump and the space between respective adjacent pits or bumps is increased gradually to the peripheral portion from the central portion of the disc. In this manner, each pit or bump at the peripheral portion of the disc is larger than that at the central portion of the disc and the space between respective adjacent pits or bumps at the peripheral portion of the disc is also larger than that at the central portion of the disc even if the same signal is recorded at both portions. Accordingly, although the contour of each pit or bump at the peripheral portion can be distinctly detected by the laser light beam tracing the spiral track, the contour of each pit or bump at the central portion can not be distinguished correctly by the laser light beam because the dimension of each of the pits or bumps thereat and the space therebetween do not become sufficiently large as compared with the diameter of the laser light beam. This means that the maximum high frequency of reproduced signal is reduced gradually to the central portion from the peripheral portion of the disc and the recorded signal at the central portion is reproduced with poor fidelity, that is, the reproduced signal, especially a reproduced video signal at the central portion is deteriorated in the quality of its frequency response and a reproduced image on a reproducing apparatus is deteriorated in resolution.